Zoom lens and imaging apparatus

ABSTRACT

A zoom lens substantially consists of a first lens group having a negative refractive power and a second lens group having a positive refractive power arranged in order from the object side; that changes magnification by moving the first lens group and the second lens group; and wherein the first lens group is configured by a first lens having a negative refractive power, a second lens, a third lens having a negative refractive power, and a fourth lens having a positive refractive power, arranged in order from the object side. The zoom lens satisfies a predetermined conditional expression.

TECHNICAL FIELD

The present invention relates to a zoom lens and more particularly to a zoom lens favorably used for small video cameras and the like.

The invention also relates to an imaging apparatus provided with such a zoom lens.

BACKGROUND ART

Heretofore, as one of the wide angle zoom lenses with a zoom ratio of about 2.5, a two-group type zoom lens formed of a first lens group having a negative refractive power and a second lens group having a positive refractive power disposed in order from the object side in which zooming is performed by moving the first and the second lens groups in optical axis directions is known. This type of zoom lens is favorably used for small video cameras and the like.

For example, U.S. Pat. No. 5,877,901 describes a two-group type zoom lens in which the first lens group is formed of four lenses, i.e., a negative lens (lens having a negative refractive power), a negative lens, a negative lens, and a positive lens (lens having a positive refractive power) disposed in order from the object side (Example 5).

U.S. Pat. No. 6,940,655 describes a two-group type zoom lens in which the first lens group is formed of four lenses, i.e., a negative lens, a positive lens, a negative lens, and a positive lens disposed in order from the object side (Example 1).

Further, U.S. Pat. No. 7,050,240 describes a two-group type zoom lens in Which the first lens group is formed of four lenses, i.e., a negative lens, a negative lens, a negative lens, and a positive lens disposed in order from the object side and the second lens group is also formed of four lenses, i.e., a positive lens, a positive lens, a negative lens, and a positive lens disposed in order from the object side (Example 1).

Moreover, U.S. Pat. No. 6,169,635 describes a two-group type zoom lens in which the first lens group is formed of four lenses, i.e., a negative lens, a positive lens, a negative lens, and a positive lens disposed in order from the object side and the second lens group is also formed of four lenses, i.e., a positive lens, a positive lens, a negative lens, and a positive lens disposed in order from the object side (Example 4).

SUMMARY OF THE INVENTION

The zoom lenses described above have the following problems. That is, the zoom lens described in U.S. Pat. No. 5,877,901 has a small zoom ratio, the zoom lens described in U.S. Pat. No. 6,169,635 has a narrow angle of view with a large F-number. The zoom lens described in U.S. Pat. No. 6,940,655 has a wide angle of view but the zoom ratio is small and the F-number is large. The zoom lens described in U.S. Pat. No. 7,050,240 has a wide angle of view with a large zoom ratio but there is still room for improvement in the distortion aberration.

The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide a zoom lens having a small F-number and a wider angle of view with well corrected aberrations.

It is a further object of the present invention to provide an imaging apparatus having satisfactory optical performance with a wider angle of view by the use of the zoom lens as described above.

A zoom lens substantially consists of a first lens group having a negative refractive power and a second lens group having a positive refractive power arranged in order from the object side, wherein:

-   zooming is performed by moving the first lens group and the second     lens group; -   the first lens group is substantially composed of a first lens     having a negative refractive power, a second lens, a third lens     having a negative refractive power, and a fourth lens having a     positive refractive power; and -   when the focal length of the second lens from the object side in the     first lens group is taken as f_(G12), and the focal length of the     entire system at the wide angle end is taken as fw, the zoom lens     satisfies a conditional expression given below:

−0.04<fw/f _(G12)<0.12  (1-1).

The term “substantially consisting of a first lens group and a second lens group” as used herein refers to include the case in which a lens having substantially no refractive power, an optical element other than a lens, such as an aperture stop, a cover glass, and the like, a lens flange, a lens barrel, an image sensor, a mechanical component, such as a camera shake correction mechanism, and the like, are included in addition to the lens groups. In this respect, the same applies to the description of “the first lens group is substantially composed of a first lens having a negative refractive power, a second lens, a third lens having a negative refractive power, and a fourth lens having a positive refractive power” and description with respect to the second lens group of “the second lens group is substantially composed of four lenses” to be discussed herein below.

In the zoom lens of the present invention, a cemented lens may be used for a lens constituting each lens group and in the case where the cemented lens is formed of n lenses cemented together, the cemented lens is counted as n lenses. Note that the term “zoom lens of the present invention” or “zoom lens according to the present invention” as used herein refers to include both of the first zoom lens and second zoom lens to be described later, unless otherwise specifically described.

In the zoom lens of the present invention, the surface shape and the sign of the refractive power of a lens are considered within the paraxial region if an aspherical surface is included.

The first zoom lens according to the present invention preferably satisfies, in particular, a conditional expression given below within the range defined by the conditional expression (1-1) with respect to the foregoing focal lengths f_(G12) and fw:

−0.01<fw/f _(G12)<0.06  (1-2).

A second zoom lens according to the present invention substantially consists of a first lens group having a negative refractive power and a second lens group having a positive refractive power, disposed in order from the object side, wherein:

-   zooming is performed by moving the first lens group and the second     lens group; -   the first lens group is substantially composed of a first lens     having a negative refractive power, a second lens, a third lens     having a negative refractive power, and a fourth lens having a     positive refractive power; -   the second lens group is substantially composed of four lenses; and -   when the focal length of the second lens from the object side in the     first lens group is taken as f_(G12), and the focal length of the     entire system at the wide angle end is taken as fw, the zoom lens     satisfies a conditional expression given below:

−0.04<fw/f _(G12)<0.17  (1-3).

The second zoom lens according to the present invention also preferably satisfies, in particular, the conditional expression (1-2) within the range defined by the conditional expression (1-3).

In the mean time, an imaging apparatus according to the present invention includes the foregoing first or second zoom lens according to the present invention.

The first zoom lens according to the present invention may inhibit increase in aberrations accompanied by increasing the angle of view while preventing cost increase by forming the first lens group with four lenses in which a first lens having a negative refractive power, a second lens, a third lens having a negative refractive power, and a fourth lens having a positive refractive power are disposed in order from the object side.

Further, the first zoom lens according to the present invention has the following advantageous effects by satisfying the conditional expression (1-1) described above. The conditional expression (1-1) defines the relationship between the focal length of the entire system at the wide angle end and the focal length of the second lens in the first lens group. If the first zoom lens falls below the lower limit value of the conditional expression (1-1), the refractive power of the second lens moves to the negative side, which leads to an imbalance between refraction of the central light beams and that of the peripheral light beams passing through the second lens, whereby correction of distortion aberration becomes difficult, which is undesirable. Contrary to this, if the first zoom lens exceeds the upper limit value of the conditional expression (1-1), the positive refractive power of the second lens becomes too strong, thereby lacking the negative refractive power of the entire first lens group, so that increasing the angle of view becomes difficult. In order to compensate for the lack of the negative refractive power of the entire first lens group, increasing the refractive power of the negative lenses within the first lens group can be considered, but if the increasing is performed, correction of aberrations becomes difficult, which is undesirable. In the case where the first zoom lens satisfies the conditional expression (1-1), the foregoing disadvantages are prevented, aberrations may be corrected satisfactorily and the increase in the angle of view can be easily achieved.

The foregoing advantageous effects become more significant if the first zoom lens satisfies the conditional expression (1-2) within the range defined by the conditional expression (1-1).

The second zoom lens according to the present invention may inhibit increase in aberrations accompanied by increasing the angle of view while preventing cost increase by forming the first lens group with four lenses in which a first lens having a negative refractive power, a second lens, a third lens having a negative refractive power, and a fourth lens having a positive refractive power are disposed in order from the object side. Further, the second zoom lens may inhibit variation in aberration due to zooming while preventing cost increase by forming the second lens group with four lenses.

Further, the second zoom lens according to the present invention has the following advantageous effects by satisfying the conditional expression (1-3). The conditional expression (1-3) defines the relationship between the focal length of the entire system at the wide angle end and the focal length of the second lens of the first lens group, as in the conditional expression (1-1). If the second zoom lens falls below the lower limit value of the conditional expression (1-3), the refractive power of the second lens moves to the negative side, which leads to an imbalance between refraction of the central light beams and that of the peripheral light beams passing through the second lens, whereby correction of distortion aberration becomes difficult, which is undesirable. Contrary to this, if the second zoom lens exceeds the upper limit value of the conditional expression (1-3), the positive refractive power of the second lens becomes too strong, thereby lacking the negative refractive power of the entire first lens group, so that increasing the angle of view becomes difficult. In order to compensate for the lack of the negative refractive power of the entire first lens group, increasing the refractive power of the negative lenses within the first lens group can be considered, but if the increasing is performed, correction of aberrations becomes difficult, which is undesirable. In the case where the second zoom lens satisfies the conditional expression (1-3), the foregoing disadvantages are prevented, aberrations may be corrected satisfactorily and increasing the angle of view can be easily achieved.

The foregoing advantageous effects become more significant if the second zoom lens satisfies the conditional expression (1-2) within the range defined by the conditional expression (1-3).

The zoom lens of the present invention may have a sufficiently small F-number, as specifically illustrated in numerical examples to be described later.

In the mean time, the imaging apparatus according to the present invention may realize favorable optical performance, low cost, and increased angle of view, as the apparatus includes the zoom lens of the present invention having the foregoing advantageous effects.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a zoom lens according to Example 1 of the present invention, illustrating the lens configuration thereof.

FIG. 2 is a cross-sectional view of a zoom lens according to Example 2 of the present invention, illustrating the lens configuration thereof.

FIG. 3 is a cross-sectional view of a zoom lens according to Example 3 of the present invention, illustrating the lens configuration thereof.

FIG. 4 is a cross-sectional view of a zoom lens according to Example 4 of the present invention, illustrating the lens configuration thereof.

FIG. 5 is a cross-sectional view of a zoom lens according to Example 5 of the present invention, illustrating the lens configuration thereof.

FIG. 6 is a cross-sectional view of a zoom lens according to Example 6 of the present invention, illustrating the lens configuration thereof.

FIG. 7 shows aberration diagrams A to H of the zoom lens according to Example 1 of the present invention.

FIG. 8 shows aberration diagrams A to H of the zoom lens according to Example 2 of the present invention.

FIG. 9 shows aberration diagrams A to H of the zoom lens according to Example 3 of the present invention.

FIG. 10 shows aberration diagrams A to H of the zoom lens according to Example 4 of the present invention.

FIG. 11 shows aberration diagrams A to H of the zoom lens according to Example 5 of the present invention.

FIG. 12 shows aberration diagrams A to H of the zoom lens according to Example 6 of the present invention.

FIG. 13 is a schematic configuration diagram of an imaging apparatus according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a cross-sectional view of a zoom lens according to an embodiment of the present invention, illustrating a configuration example, which corresponds to a zoom lens of Example 1 to be described later. FIGS. 2 to 6 are cross-sectional views, illustrating other configuration examples, each corresponding to each of zoom lenses of Example 2 to 6. The basic configurations of the examples illustrated in FIGS. 1 to 6 are identical to each other, except for a particularly described difference, and the methods of illustration are also identical, so that the zoom lens according to an embodiment of the present invention will be described mainly with reference to FIG. 1.

In FIG. 1, the left side is the object side and the right side is the image side, in which the diagram A illustrates the arrangement of the optical system in infinity focusing state at the wide angle end (state of the shortest focal length) while the diagram B illustrates the arrangement of the optical system in infinity focusing state at the telephoto end (state of the longest focal length) Note that the same applies to FIGS. 2 to 6, to be described later.

The zoom lens according to an embodiment of the present invention is formed of a first lens group G1 having a negative refractive power and a second lens group G2 having a positive refractive power disposed in order from the object side as lens groups. Further, a fixed aperture stop St, which does not move during zooming, is disposed between the first lens group G1 and the second lens group G2. The aperture stop St shown here does not necessarily represent the size and shape but rather indicates the position on the optical axis Z.

Note that FIG. 1 illustrates an example in which a parallel plate optical member PP is disposed between the second lens group G2 and the image plane Sim. When applying the zoom lens to an imaging apparatus, it is preferable that a cover glass and various types of filters, such as a low-pass filter, an infrared cut filter, and the like, are disposed between the optical system and the image plane Sim according to the structure on the camera side on which the zoom lens is mounted. The optical member PP represents such a cover glass and various types of filters. Further, some imaging devices in recent years employ a three-CCD system that uses a CCD for each color in order to improve image quality, and a color separation optical system, such as a color separation prism or the like, will be inserted between the lens system and the image plane Sim in order to accommodate the three-CCD system. If that is the case, the color separation optical system may be disposed at the position of the optical member PP.

The zoom lens is formed such that, upon zooming from the wide angle end to the telephoto end, the first lens group G1 is moved to the side of the image plane Sim so as to draw a convex trajectory while the second lens group G2 is moved monotonically to the object side. FIG. 1 schematically illustrates the movement trajectories of the first lens group G1 and second lens group G2 upon zooming from the wide angle end to the telephoto end by solid lines between the diagrams A and B.

The first lens group G1 is formed of four lenses in which a first lens L11 having a negative refractive power, a second lens L12 having a positive refractive power, a third lens L13 having a negative refractive power, and a fourth lens L14 baying a most refractive power are disposed in order from the object side. Here, for example, the first lens L11 may have a negative meniscus shape, the second lens L12 may have an aspherical shape on both the object side surface and the image side surface, the third lens L13 may have a negative meniscus shape, and the fourth lens L14 may have a positive meniscus shape, as in the example shown in FIG. 1. Note that a lens having a negative refractive power is used as the second lens L12 particularly in Example 4.

The object side surface of the second lens L12 is formed in an aspherical surface with a concave surface on the object side in the paraxial region. Further, at least either one of the object side surface and the image side surface both surfaces in the example of FIG. 1) is formed in an aspherical surface having at least one inflection point on the surface from the center to the effective diameter. Note that, particularly in Example 2, the object side surface of the second lens L12 is formed in an aspherical surface with a convex surface on the object side in the paraxial region and without any inflection point on the surface from the center to the effective diameter.

In the mean time, the second lens group G2 is formed of four lenses in which a first lens L21 having a positive refractive power, a second lens L22 having a positive refractive power, a third lens L23 having a negative refractive power, and a fourth lens L24 having a positive refractive power are disposed in order from the object side. Here, for example, the first lens L21 may have an aspherical shape on both the object side surface and the image side surface, the second lens L22 may have a biconvex shape, the third lens L23 may have a negative meniscus shape, and the fourth lens L24 may have a biconvex shape, as in the example shown in FIG. 1.

As described above, in the present zoom lens, the first lens group G1 is formed of four lenses in which the first lens L11 having a negative refractive power, the second lens L12, the third lens L13 having a negative refractive power, and the fourth lens L14 having a positive refractive power are disposed in order from the object side. This may prevent increasing aberrations accompanied by increasing the angle of view while preventing cost increase. Further, in Examples other than Example 4, distortion aberration may be corrected satisfactorily by the use of a lens having a positive refractive power as the second lens L12.

As the second lens L12 of the first lens group GI has an aspherical surface on the object side surface, distortion aberration may be corrected satisfactorily, and the cost of the zoom lens may be kept low in comparison with the case in which the first lens L11 is formed in an aspherical surface. That is, the positions where the axial light beam and of light beam pass through are generally separated largely before and after the first lens L11, and the first lens L11 or the second lens L12 is preferably an aspherical lens in order to satisfactorily correct distortion aberration. As the first lens L11 generally has a relatively large diameter, if the second lens L12 having relatively a small diameter is formed in an aspherical surface, the cost of the aspherical lens is reduced and eventually the cost of the zoom lens may be kept low.

In Examples other than Example 2, the object side surface of the second lens L12 is formed in an aspherical surface with a concave surface on the object side, in particular, in the paraxial region, so that spherical aberration and distortion aberration may be corrected satisfactorily.

Further, in Examples other than Example 2, at least either one of the object side surface and the image side surface of the second lens L12 is formed in an aspherical surface having at least one inflection point on the surface from the center to the effective diameter, so that distortion aberration and field curvature at the wide angle end may be corrected satisfactorily.

In the mean time, the formation of second lens group G2 with four lenses may inhibit variation in aberration due to zooming while preventing cost increase.

In the present zoom lens, the second lens group G2 is formed of four lenses in which the first lens L21 having a positive refractive power, the second lens L22 having a positive refractive power, the third lens L23 having a negative refractive power, and the fourth lens having a positive power are disposed in order from the object side. This may inhibit variation in aberration due to zooming. That is, if the first lens L21 and the second lens L22 of the second lens group G2 are positive lenses, the axial light beam outputted from the first lens group G1 and diffused largely may be converged by the two positive lenses L21 and L22 in a shared manner, so that the high order spherical aberration may be kept small and variation in aberration due to zooming may be inhibited.

Here, when the focal length of the second lens L12 which is the second lens from the object side in the first lens group G1 is taken as f_(G12) and the focal length of the entire system at the wide angle end is taken as fw, the present zoom lens satisfies all of the aforementioned conditional expressions:

−0.04<fw/f _(G12)<0.12  (1-1);

−0.01<fw/f _(G12)<0.06  (1-2); and

−0.04<fw/f _(G12)<0.17  (1-3).

Numerical value examples of conditions defined by the foregoing conditional expressions are summarized in Table 19 with respect to each Example. The values of fw/f_(G12) defined by each of the conditional expression are indicated in the row of “Conditional Expression (1)”. Further, the Table 19 also indicates numerical value examples of each condition defined by conditional expressions (2) to (9), to be described later.

The operation and advantageous effects of the configurations defined by the conditional expressions (1-1), (1-2) and (1-3) will now be described.

The conditional expressions (1-1), (1-2) and (1-3) define the relationship between the focal length of the entire system at the wide angle end and the focal length of the second lens L12 of the first lens group G1. After the first lens group G1 is formed of the first lens L11 having a negative refractive power, the second lens L12, the third lens L13 having a negative refractive power, and the fourth lens L14 having a positive refractive power, as described above, the present zoom lens satisfies the conditional expression (1-1), thereby obtaining the following advantageous effects. That is, if the zoom lens falls below the lower limit value of the conditional expression (1-1), the refractive power of the second lens L12 moves to the negative side, which leads to an imbalance between refraction of the central light beans and that of the peripheral light beams passing through the second lens L12, whereby correction of distortion aberration becomes difficult, which is undesirable. Contrary to this, if the zoom lens exceeds the upper limit value of the conditional expression (1-1), the positive refractive power of the second lens L12 becomes too strong, thereby lacking the negative refractive power of the entire first lens group G1, so that increasing the wide angle of view becomes difficult. In order to compensate for the lack of the negative refractive power of the entire first lens group G1, increasing the refractive power of the negative lenses within the first lens group G1, i.e., the first lens L11 and the third lens L13, can be considered, but if the increasing is performed, correction of aberrations becomes difficult, which is undesirable. In the case where the zoom lens satisfies the conditional expression (1-1), the foregoing disadvantages are prevented, aberrations may be corrected satisfactorily and increasing the angle of view can be easily achieved.

The foregoing advantageous at become more significant since the present zoom lens satisfies, in particular the conditional expression (1-2) within the range defined by the conditional expression (1-1).

In the present zoom lens, after the first lens group G1 is formed of the first lens L11 having a negative refractive power, the second lens L12, the third lens L13 having a negative refractive power, and the fourth lens L14 having a positive refractive power, as described above, and the second lens group G2 is formed of four lenses, the present zoom lens satisfies the conditional expression (1-3), thereby obtaining the following advantageous effects. That is, if the zoom lens falls below the lower limit value of the conditional expression (1-3), the refractive power of the second lens L12 moves to the negative side, which leads to an imbalance between refraction of the central light beams and that of the peripheral light beams passing through the second lens L12, whereby correction of distortion aberration becomes difficult, which is undesirable. Contrary to this, if the zoom lens exceeds the upper limit value of the conditional expression (1-3), the positive refractive power of the second lens L12 becomes too strong, thereby lacking the negative refractive power of the entire first lens group G1, so that increasing the wide angle of view becomes difficult. In order to compensate for the lack of the negative refractive power of the entire first lens group G1, increasing the refractive power of the negative lenses within the first lens group G1, i.e., the first lens L11 and the third lens L13, can be considered, but if the increasing is performed, correction of aberrations becomes difficult, which is undesirable. In the case where the first zoom lens satisfies the conditional expression (1-3), the foregoing disadvantages are prevented, aberrations may be corrected satisfactorily and increasing the angle of view can be easily achieved.

The foregoing advantageous effects become more significant since the present zoom lens satisfies, in particular the conditional expression (1-2) within the range defined by the conditional expression (1-3).

The present zoom lens satisfies the conditional expression given below when the focal length of the entire system at the wide angle end is taken as fw and the focal length of the first lens group G1 is taken as f₁, thereby obtaining the following advantageous effects:

0.00<|fw/f ₁|<0.64  (2).

That is, this conditional expression (2) defines the relationship between the focal length fw of the entire system at the wide angle end and the focal length f₁ of the first lens group G1. If the zoom lens exceeds the upper limit value of the conditional expression (2), the negative refractive power of the first lens group G1 becomes too strong and the correction of off-axis aberrations becomes difficult, which is undesirable. In the case where the zoom lens satisfies the conditional expression (2), the foregoing shortcomings are prevented and off-axis aberrations may be corrected easily.

If the zoom lens satisfies, in particular, a conditional expression given below within the range defined by the conditional expression (2), the foregoing advantageous effects become more significant:

0.20<|fw/f ₁|<0.50  (2′).

If the zoom lens falls below the lower limit value of the conditional expression (2′), the negative refractive power of the first lens group G1 becomes weak and the entire optical system becomes large, which is undesirable. In the case where the zoom lens satisfies the conditional expression (2′), the foregoing shortcomings are prevented and downsizing of the entire optical system may be achieved.

When the focal length of the entire system at the wide angle end is taken as fw and the focal length of the second lens group G2 is taken as f₂, the present zoom lens satisfies a conditional expression given below, so that the following advantageous effects may also be obtained:

0.31<fw/f ₂<0.49  (3).

That is, the conditional expression (3) defines the relationship between the focal length fw of the entire system at the wide angle end and the focal length f₂ of the second lens group G2. If the zoom lens falls below the lower limit value of the conditional expression (3), the refractive power of the second lens group G2 becomes weak and the amount of movement of the second lens group G2 is increased at the time of zooming, thereby resulting in an extended overall length of the entire optical system and downsizing becomes difficult, which is undesirable, Contrary to this, if the zoom lens exceeds the upper limit value of the conditional expression (3), the refractive power of the second lens group G2 becomes too strong and satisfactory correction of aberration over the entire zoom range becomes difficult, which is undesirable. In the case where the zoom lens satisfies the conditional expression (3), the foregoing disadvantages are prevented and downsizing of the entire optical system may be achieved, as well as easy correction of aberrations over the entire zoom range.

In the case that the present zoom lens satisfies, in particular, the conditional expression (3′) given below within the range defined by the conditional expression (3), the foregoing advantageous effects become more significant:

0.31<fw/f ₂<0.35  (3′).

When the focal length of the first lens group G1 is taken as f₁ and the focal length of the second lens group G2 is taken as f₂, the present zoom lens satisfies a conditional expression given below, so that the following advantageous effects may also be obtained:

0.56<|f ₁ /f ₂|<1.04  (4).

That is, the conditional expression (4) defines the relationship between the focal length f₁ of the first lens group G1 and the focal length f₂ of the second lens group G2. If the zoom lens falls below the lower limit value of the conditional expression (4), the refractive power of the second lens group G2 becomes weak and the amount of movement of the second lens group G2 is increased at the time of zooming, thereby resulting in an extended overall length of the entire optical system and downsizing becomes difficult, which is undesirable. Contrary to this, if the zoom lens exceeds the upper limit value of the conditional expression (4), the refractive power of the first lens group G1 becomes insufficient and the diameter of the first lens L11 located on the most object side needs to be increased in order to secure the angle of view, whereby downsizing becomes difficult, which is undesirable. In the case where the zoom lens satisfies the conditional expression (4), the foregoing disadvantages are prevented and the entire optical system may be downsized easily.

In the case where zoom lens satisfies, in particular, a conditional expression given below within the range defined by the conditional expression (4), the foregoing advantageous effects become more significant:

0.70<|f ₁ /f ₂|<0.80  (4′).

Still further, when the focal length of the first lens group G1 is taken as f₁ and the focal length of the second lens from the object side in the first lens group G1 is taken as f_(G12), the present zoom lens satisfies a conditional expression given below, so that the following advantageous effects may also be obtained:

−0.19<|f ₁ /f _(G12)|<0.50  (5).

That is, the conditional expression (5) defines the relationship between the focal length f₁ of the first lens group G1 and the focal length f_(G12) of the second lens L12 of the first lens group G1. If the zoom lens falls below the lower limit value of the conditional expression (5), the positive refractive power of the second lens L12 becomes too strong and the refractive power of a lens (the first lens L11 or the third lens L13) having a negative refractive power in the first lens group G1 becomes too strong in order to compensate for this, whereby correction of aberrations becomes difficult, which is undesirable. Contrary to this, if the zoom lens exceeds the upper limit value of the conditional expression (5), the negative refractive power of the second lens L12 becomes too strong and correction of distortion aberration becomes difficult, which is undesirable. In the case where the zoom lens satisfies the conditional expression (5), the foregoing shortcomings are prevented and distortion aberration and other aberrations may be corrected satisfactorily.

The foregoing advantageous effects become more significant if the zoom lens satisfies, in particular, the conditional expression (5′) given below within the range defined by the conditional expression (5):

−0.15<|f ₁ /f _(G12)|<0.30  (5′).

when the maximum effective radius of the object side surface of the second lens from the object side in the first lens group G1 is taken as H_(G12F), the radius of curvature of a spherical surface passing through the center of the object side surface of the second lens and a point on the object side surface at a height of H_(G12F) from the optical axis with the center of the object side surface as the apex of the spherical surface is taken as r′_(G12F), and the radius of curvature of a spherical surface passing through the center of the object side surface of the second lens and a point on the object side surface at a height of H_(G12F)×0.5 from the optical axis with the center of the object side surface as the apex of the spherical surface is taken as r″_(G12F), the present zoom lens satisfies a conditional expression given below, so that the following advantageous effects may also be obtained:

0.20<H _(G12F)×{(1/r′ _(G12F))−(1/r″ _(G12F))}  (6).

That is, the conditional expression (6) defines, with respect to the object side surface of the second lens L12 in the first lens group G1, the relationship between the maximum effective radius and the aspherical shape. By giving a difference in the radius of curvature between the vicinity of the center of the object side surface of the second lens L12 and the periphery within the range defined by the conditional expression (6), distortion aberration at the wide angle end may be corrected satisfactorily. If the zoom lens falls below the lower limit value of the conditional expression (6), the distortion aberration is under-corrected while if it exceeds the upper limit value, the distortion aberration is over-corrected, either of which is undesirable.

In the case where the zoom lens satisfies, in particular, a conditional expression given below within the range defined by the conditional expression (6), the foregoing advantageous effects become more significant:

0.20<H _(G12F)×{(1/r′ _(G12F))−(1/r″ _(G12F))}<0.50  (6′).

Still further when the paraxial radius of curvature of the object side surface of the second, lens from the object side in the first lens group G1 is taken as r_(G12F) and the paraxial radius of curvature of the image side surface of the second lens from the object side in the first lens group G1 is taken as r_(G12R), the present zoom lens satisfies a conditional expression given below, so that the following advantageous effects may also be obtained:

2.0<(r _(G12F) +r _(G12R))/(r _(G12F) −r _(G12R))<30.0  (7).

That is, the conditional expression (7) defines the shape of the second lens L12 of the first lens group G1. If the zoom lens fails below the lower limit value of the conditional expression (7), distortion aberration is under corrected on the wide angle end side, which is undesirable. Contrary to this, if the zoom lens exceeds the upper limit value, satisfactory correction of spherical aberration becomes difficult on the telephoto end side, which is undesirable. In the case where the zoom lens satisfies the conditional expression (7), the foregoing shortcomings are prevented and distortion aberration on the wide angle end side and spherical aberration on the telephoto end side may be corrected satisfactorily.

In the case where the zoom lens satisfies, in particular, a conditional expression given below within the range defined by the conditional expression (7), the foregoing advantageous effects becomes more significant:

2.0<(r _(G12F) +r _(G12R))/(r _(G12F) −r _(G12R))<15.0  (7′).

Further, when the paraxial radius of curvature of the object side surface of the first lens from the object side in the first lens group G1 is taken as r_(G11P) and the paraxial radius of curvature of the image side surface of the first lens from the object side in the first lens group G1 is taken as r_(G11R), the present zoom lens satisfies a conditional expression given below, so that the following advantageous effects may also be obtained:

2.5<(r _(G11P) +r _(G11R))/(r _(G11P) −r _(G11R))<10.0  (8).

That is, the conditional expression (8) defines the shape of the first lens L11 in the first lens group G1. If the zoom lens falls below the lower limit value of the conditional expression (8), field curvature is under corrected on the wide angle end side, which is undesirable. Contrary to this, if the zoom lens exceeds the upper limit value, field curvature is over corrected on the wide angle end side, which is undesirable. In the case where the zoom lens satisfies the conditional expression (8), the foregoing shortcomings are prevented and field curvature on the wide angle end side may be corrected appropriately.

In the case where the zoom lens satisfies, in particular, a conditional expression given below within the range defined by the conditional expression (8), the foregoing advantageous effects become more significant:

2.8<(r _(G11P) +r _(G11R))/(r _(G11P) −r _(G11R))<4.0  (8′).

Still further, when the focal length of the first lens from the object side in the second lens group is taken as f_(G21) and the focal length of the second lens from the object side in the second lens group G2 is taken as f_(G22), the present zoom lens satisfies a conditional expression given below, so that the following advantageous effects may also be obtained:

1.3<f _(G21) /f _(G22)<3.0  (9).

That is, the conditional expression (9) defines, with respect to the first lens L21 and the second lens L22 in the second lens group G2, the relationship between their focal lengths. If the zoom lens falls below the lower limit value of the conditional expression (9), spherical aberration is under corrected, which is undesirable. Contrary to this, if the zoom lens exceeds the upper limit value, spherical aberration is over corrected, which is undesirable. In the case where the zoom lens satisfies the conditional expression (9), the foregoing shortcomings are prevented and spherical aberration may be corrected satisfactorily over the entire zoom range.

In the case where the zoom lens satisfies, in particular, a conditional expression given below within the range defined by the conditional expression (9), the foregoing advantageous effects becomes more significant:

2.0<f _(G21) /f _(G22)<2.5  (9′).

FIG. 1 illustrates an example case in which an optical member PP is disposed between the lens system and image forming surface but, instead of disposing various types of filters, such as a low-pass filter, a filter that will cut a particular wavelength range, and the like as the optical member PP, the filters may be disposed between each lens or a coating having an identical effect to that of the filter may be provided on a lens surface of any of the lenses.

Numerical examples of the zoom lens of the present invention will now be described. The lens cross-sectional views of zoom lenses of Examples 1 to 6 are shown in FIGS. 1 to 6 respectively.

Further, basic lens data, zoom data, and aspherical surface data of the zoom lens of Example 1 are shown in Tables 1, 2, and 3 respectively. Likewise, basic lens data, zoom data, and aspherical surface data of zoom lenses of Examples 2 to 6 are shown in Tables 4 to 18. Meanings of the symbols in the tables will be described herein below, by taking those of Example 1 as example, but the same applies basically to Examples 2 to 6.

In the lens data shown in Table 1, the Si column indicates i^(th) surface number in which a number i (i=1, 2, 3, - - - ) is given to each surface in a serially increasing manner toward the image side with the most object side surface being taken as the first surface. The Ri column indicates the radius of curvature of i^(th) surface and the Di column indicates the surface distance between i^(th) surface and (i+1)^(th) surface on the optical axis Z. Note that the sign of the radius of curvature is positive if the surface shape is convex on the object side and negative if it is convex on the image side.

Further, in the basic lens data, the Ndj column indicates the refractive index of j^(th) optical element with respect to the d-line (587.6 nm) in which a number j (j=1, 2, 3, - - - ) is given to each optical element in a serially increasing manner toward the image side with the optical element on the most object side being taken as the first optical element, and the vdj column indicates the Abbe number of j^(th) optical element with respect to the d-line. Note that the basic lens data shown include an aperture stop St and “∞ (Aperture)” is indicated in the column of radius of curvature of the surface corresponding to the aperture stop St.

The symbols D8, D9, and D17 in the basic lens data are surface distances that will vary at the time of zooming, in which D9 represents the distance between the aperture stop St and second lens group G2 while D17 represents the distance between the second lens group G2 and the optical member PP.

The zoom data shown in Table 2 indicate focal length (f) of the entire system, F-number (Fno.), total angle of view (2ω), and value of each surface distance that will vary at the time of zooming at each of the wide angle end and the telephoto end.

In the lens data shown in Table 1, an asterisk mark * is attached to the surface number of an aspherical surface and a value of paraxial radius of curvature is shown as the radius of curvature of the aspherical surface. The aspherical surface data shown in Table 3 indicate surface numbers of aspherical surfaces and aspherical surface coefficients of each aspherical surface. The value “E−n” (n: integer) represents “×10^(−n)”. The aspherical surface coefficients represent values of coefficients NA and RAm (m=3, 4, 5, - - - 16) in an aspherical surface expression given below:

Zd=C·h ²/{1+(1−Ka·C ² ·h ²)^(1/2) }+ΣRAm·h ^(m),

where

Zd: depth of aspheric surface (length of vertical line from a point on the aspheric surface at height h to a flat surface orthogonal to the optical axis to which the aspherical apex contacts);

h: height (distance from the optical axis to lens surface);

C: inverse of paraxial radius of curvature; and

KA, RAm: aspherical surface coefficients (m=3, 4, 5, - - - 16). Values rounded to a predetermined digit are shown in the following tables. Further, in the data of the table shown below, degree is used as the unit of angle and ram is used as the unit of length, but other appropriate units may also be used, as optical systems are usable even when they are proportionally increased or decreased.

TABLE 1 Example 1 • Basic Lens Data Si Ri Di Ndj νdj (Surface (Radius of (Surface (Refractive (Abbe Number) Curvature) Distance) Index) Number) 1 16.7910 0.80 1.78590 44.2 2 8.7843 3.04 *3 −22.1777 2.10 1.53389 56.0 *4 −18.3950 0.67 5 158.3861 0.70 1.78590 44.2 6 5.9611 2.50 7 8.1910 1.53 1.92286 18.9 8 11.8859 D8  9 ∞ (Aperture) D9  *10 11.4416 1.50 1.53389 56.0 *11 58.5954 0.10 12 9.4968 4.15 1.49700 81.5 13 −11.2458 0.90 14 14.6399 0.70 1.92286 20.9 15 6.0474 1.02 16 17.2969 2.25 1.51742 52.4 17 −15.0096 D17 18 ∞ 1.01 1.51633 64.1 19 ∞ 6.84 *Aspherical Surface

TABLE 2 Example 1 • Zoom Data Specs Wide Angle End Telephoto End f 3.18 7.95 Fno. 1.85 3.10 2ω 93.39 43.28 D8 12.10 3.55 D9 7.13 0.96 D17 0.00 6.17

TABLE 3 Example 1 • Aspherical Surface Data Surface Number S3 S4 KA 1.00000000E+00 1.00000000E+00 RA3 3.92552657E−04 −1.78198417E−03 RA4 1.63491671E−03 2.96047622E−03 RA5 −5.98243470E−05 −3.54470466E−04 RA6 −3.12580573E−05 −2.14656523E−05 RA7 3.08631891E−06 3.49680699E−06 RA8 2.06084921E−07 5.77269401E−07 RA9 −3.30656971E−08 1.80867183E−08 RA10 7.50984913E−10 −1.28540306E−08 RA11 4.80884982E−10 −1.51109077E−09 RA12 −6.15184533E−11 2.22386867E−10 Surface Number S10 S11 KA 1.00000000E+00 1.00000000E+00 RA3 1.88211972E−03 1.76860217E−03 RA4 −1.21236781E−03 −2.69165382E−04 RA5 6.04426291E−04 3.95866507E−04 RA6 −8.55374397E−05 −2.23064469E−05 RA7 −4.99070718E−06 −9.52288260E−06 RA8 6.90562953E−07 1.17774794E−06 RA9 1.79754879E−07 −6.42044665E−08 RA10 4.73691904E−09 6.39130198E−09 RA11 −4.62119417E−10 3.66073819E−09 RA12 −2.98496187E−10 5.76274981E−11 RA13 3.48467387E−11 −1.74712784E−10 RA14 −1.45151464E−11 3.01771364E−11 RA15 −3.10163706E−12 −4.20522148E−13 RA16 3.84723135E−13 −7.01830246E−13

TABLE 4 Example 2 • Basic Lens Data Si Ri Di Ndj νdj (Surface (Radius of (Surface (Refractive (Abbe Number) Curvature) Distance) Index) Number) 1 12.0000 0.85 1.83481 42.7 2 7.7547 3.00 *3 333.8853 2.40 1.53389 56.0 *4 −188.2116 2.26 5 −111.2925 0.70 1.88300 40.8 6 5.9612 1.45 7 7.9606 1.85 1.92286 18.9 8 15.3951 D8 9 ∞ (Aperture) D9 *10 14.8790 2.00 1.53389 56.0 *11 −27.1649 0.47 12 11.7962 4.30 1.61800 63.3 13 −9.3009 0.10 14 −72.9131 0.70 1.84666 23.8 15 7.0342 0.75 16 18.7928 2.20 1.58144 40.8 17 −14.1574 D17 18 ∞ 1.01 1.51633 64.1 19 ∞ 6.82 *Aspherical Surface

TABLE 5 Example 2 • Zoom Data Specs Wide Angle End Telephoto End f 3.36 8.39 Fno. 1.82 3.18 2ω 90.62 41.16 D8 8.77 2.45 D9 8.58 1.94 D17 1.00 7.64

TABLE 6 Example 2 • Aspherical Surface Data Surface Number S3 S4 KA 1.00000000E+00 1.00000000E+00 RA3 −1.84518151E−04 −1.34452536E−03 RA4 9.11020231E−04 1.50433453E−03 RA5 4.36869407E−05 −1.88326554E−05 RA6 −9.99470488E−06 −1.02662481E−05 RA7 −4.77789164E−07 −1.12222737E−06 RA8 3.77729589E−08 −8.06437604E−08 RA9 7.69951469E−09 1.03269414E−09 RA10 1.57751409E−09 1.07327708E−09 RA11 −1.53152663E−10 1.29361362E−10 RA12 −1.25879264E−11 3.43709353E−12 Surface Number S10 S11 KA 1.86689146E+00 −2.34761165E+00 RA3 6.38886087E−04 8.63959607E−04 RA4 −1.72624605E−04 4.82943361E−04 RA5 1.56425637E−04 1.17579182E−04 RA6 −1.72640984E−05 1.95121845E−05 RA7 −1.15229550E−06 −7.32560718E−06 RA8 4.72324927E−08 1.36463243E−06 RA9 6.56977631E−08 −6.02693745E−08 RA10 −6.88442793E−09 −3.61578440E−10 RA11 −2.38022413E−10 −1.02638686E−10 RA12 −3.22383884E−11 −6.75284248E−12 RA13 1.02907272E−12 3.06525203E−12 RA14 5.19740494E−13 2.84165637E−12 RA15 −1.83177171E−13 −4.24793926E−13 RA16 −1.74412406E−16 5.14814396E−14

TABLE 7 Example 3 • Basic Lens Data Si Ri Di Ndj νdj (Surface (Radius of (Surface (Refractive (Abbe Number) Curvature) Distance) Index) Number) 1 18.0197 0.80 1.78590 44.2 2 8.8085 3.13 *3 −29.3048 2.54 1.53389 56.0 *4 −15.3177 0.26 5 −387.3951 0.70 1.78590 44.2 6 5.9157 2.44 7 7.9344 1.56 1.92286 18.9 8 11.3636 D8 9 ∞ (Aperture) D9 *10 11.4802 1.50 1.53389 56.0 *11 59.6824 0.10 12 9.5074 4.20 1.49700 81.5 13 −11.0673 0.92 14 14.9169 0.74 1.92286 20.9 15 6.0354 0.95 16 17.4298 2.23 1.51742 52.4 17 −14.7168 D17 18 ∞ 1.01 1.51633 64.1 19 ∞ 6.79 *Aspherical Surface

TABLE 8 Example 3 • Zoom Data Specs Wide Angle End Telephoto End f 3.19 7.98 Fno. 1.84 3.10 2ω 93.23 43.22 D8 12.05 3.55 D9 7.10 0.95 D17 0.00 6.16

TABLE 9 Example 3 • Aspherical Surface Data Surface Number S3 S4 KA 1.00000000E+00 1.00000000E+00 RA3 −3.92896399E−04 −2.13763767E−03 RA4 1.59073904E−03 2.91750862E−03 RA5 −5.69315036E−05 −3.58929668E−04 RA6 −3.09012532E−05 −2.16238082E−05 RA7 3.08376455E−06 3.52532145E−06 RA8 2.01913214E−07 5.85035760E−07 RA9 −3.35542117E−08 1.83747727E−08 RA10 7.17802063E−10 −1.27915817E−08 RA11 4.82375497E−10 −1.50321640E−09 RA12 −6.07407734E−11 2.23818828E−10 Surface Number S10 S11 KA 1.00000000E+00 1.00000000E+00 RA3 1.76132207E−03 1.62917632E−03 RA4 −1.20250122E−03 −2.54326990E−04 RA5 6.05031687E−04 3.97279047E−04 RA6 −8.55614525E−05 −2.21530506E−05 RA7 −4.99565629E−06 −9.51075191E−06 RA8 6.90298187E−07 1.17877317E−06 RA9 1.79779961E−07 −6.41427571E−08 RA10 4.74445204E−09 6.39564855E−09 RA11 −4.60842095E−10 3.66096527E−09 RA12 −2.98218247E−10 5.76689611E−11 RA13 3.48935761E−11 −1.74705742E−10 RA14 −1.45031348E−11 3.01805056E−11 RA15 −3.10067265E−12 −4.18867888E−13 RA16 3.84662428E−13 −7.01204898E−13

TABLE 10 Example 4 • Basic Lens Data Si Ri Di Ndj νdj (Surface (Radius of (Surface (Refractive (Abbe Number) Curvature) Distance) Index) Number) 1 15.0647 0.80 1.78590 44.2 2 8.7870 3.39 *3 −12.0041 1.68 1.53389 56.0 *4 −13.2378 0.72 5 79.6843 0.70 1.83481 42.7 6 6.0492 2.40 7 8.3918 1.57 1.92286 18.9 8 12.8384 D8 9 ∞ (Aperture) D9 *10 11.5886 1.50 1.53389 56.0 *11 62.6674 0.10 12 9.3886 4.14 1.49700 81.5 13 −11.4819 0.84 14 14.3873 0.70 1.92286 20.9 15 6.0411 1.06 16 18.2998 2.26 1.51742 52.4 17 −14.5710 D17 18 ∞ 1.01 1.51633 64.1 19 ∞ 6.91 *Aspherical Surface

TABLE 11 Example 4 • Zoom Date Specs Wide Angle End Telephoto End f 3.19 7.99 Fno. 1.85 3.10 2ω 93.11 43.14 D8 12.11 3.55 D9 7.15 0.96 D17 0.00 6.19

TABLE 12 Example 4 • Aspherical Surface Data Surface Number S3 S4 KA 1.00000000E+00 1.00000000E+00 RA3 1.97673389E−03 −5.14428429E−04 RA4 1.87516095E−03 3.12292451E−03 RA5 −8.83901056E−05 −3.73387017E−04 RA6 −3.46164616E−05 −2.15361781E−05 RA7 3.39216521E−06 3.59181888E−06 RA8 2.83246128E−07 6.11999137E−07 RA9 −3.25234250E−08 2.16820702E−08 RA10 −1.62438093E−10 −1.26000740E−08 RA11 4.02161038E−10 −1.56205805E−09 RA12 −4.93251419E−11 2.07287653E−10 Surface Number S10 S11 KA 1.00000000E+00 1.00000000E+00 RA3 1.94128295E−03 1.86623411E−03 RA4 −1.23941301E−03 −3.36191383E−04 RA5 6.08149148E−04 4.06331604E−04 RA6 −8.51668505E−05 −2.18876430E−05 RA7 −4.93462600E−06 −9.60813947E−06 RA8 6.94019051E−07 1.17663239E−06 RA9 1.80217543E−07 −6.42807894E−08 RA10 4.56975362E−09 6.62207183E−09 RA11 −4.56633408E−10 3.67837311E−09 RA12 −2.91798675E−10 6.28058891E−11 RA13 3.76317402E−11 −1.70699217E−10 RA14 −1.38781021E−11 3.16879777E−11 RA15 −2.99033991E−12 2.08736136E−13 RA16 3.59472958E−13 −8.60992007E−13

TABLE 13 Example 5 • Basic Lens Data Si Ri Di Ndj νdj (Surface (Radius of (Surface (Refractive (Abbe Number) Curvature) Distance) Index) Number) 1 17.7205 0.80 1.78590 44.2 2 8.7860 3.01 *3 −36.6744 2.61 1.53389 56.0 *4 −19.6099 0.39 5 421.7536 0.70 1.78590 44.2 6 5.9262 2.47 7 8.0207 1.54 1.92286 18.9 8 11.4973 D8 9 ∞ (Aperture) D9 *10 11.3062 1.50 1.53389 56.0 *11 55.2334 0.10 12 9.4789 4.16 1.49700 81.5 13 −11.2650 0.92 14 14.8237 0.70 1.92286 20.9 15 6.0417 0.94 16 16.2485 2.19 1.51742 52.4 17 −15.4996 D17 18 ∞ 1.01 1.51633 64.1 19 ∞ 6.85 *Aspherical Surface

TABLE 14 Example 5 • Zoom Data Specs Wide Angle End Telephoto End f 3.20 7.99 Fno. 1.85 3.10 2ω 93.25 43.15 D8 12.04 3.55 D9 7.09 0.95 D17 0.00 6.14

TABLE 15 Example 5 • Aspherical Surface Data Surface Number S3 S4 KA 1.00000000E+00 1.00000000E+00 RA3 −4.30601440E−04 −2.45942098E−03 RA4 1.43624994E−03 2.89828666E−03 RA5 −3.55884451E−05 −3.71151955E−04 RA6 −3.08553414E−05 −2.14177604E−05 RA7 2.83817696E−06 3.61955608E−06 RA8 1.79586380E−07 5.91009605E−07 RA9 −3.24112553E−08 1.81447671E−08 RA10 1.20108913E−09 −1.28743984E−08 RA11 5.15204525E−10 −1.51244540E−09 RA12 −6.86137874E−11 2.22680423E−10 Surface Number S10 S11 KA 1.00000000E+00 1.00000000E+00 RA3 1.76814202E−03 1.62203935E−03 RA4 −1.21678337E−03 −2.45116314E−04 RA5 6.06179046E−04 3.88478822E−04 RA6 −8.58569578E−05 −2.21235612E−05 RA7 −4.99796243E−06 −9.42747893E−06 RA8 6.97601547E−07 1.19048864E−06 RA9 1.81003252E−07 −6.30576600E−08 RA10 4.91247208E−09 6.49947884E−09 RA11 −4.43304548E−10 3.66517494E−09 RA12 −2.94072014E−10 5.85100526E−11 RA13 3.45122935E−11 −1.74157715E−10 RA14 −1.43374070E−11 2.94645898E−11 RA15 −3.18348899E−12 −4.90574065E−13 RA16 3.91257516E−13 −6.76391292E−13

TABLE 16 Example 6 • Basic Lens Data Si Ri Di Ndj νdj (Surface (Radius of (Surface (Refractive (Abbe Number) Curvature) Distance) Index) Number) 1 17.9420 0.80 1.78590 44.2 2 8.7868 2.94 *3 −70.8941 2.93 1.53389 56.0 *4 −26.6446 0.37 5 400.8261 0.70 1.78590 44.2 6 5.8867 2.44 7 8.1404 1.54 1.92286 18.9 8 11.8520 D8 9 ∞ (Aperture) D9 *10 11.2098 1.50 1.53389 56.0 *11 52.9915 0.10 12 9.2969 4.14 1.49700 81.5 13 −11.5666 0.87 14 14.2844 0.70 1.92286 20.9 15 5.9671 0.95 16 15.0986 2.25 1.51742 52.4 17 −16.6844 D17 18 ∞ 1.01 1.51633 64.1 19 ∞ 6.81 *Aspherical Surface

TABLE 17 Example 6 • Zoom Data Specs Wide Angle End Telephoto End f 3.18 7.95 Fno. 1.84 3.10 2ω 93.44 43.18 D8 11.88 3.55 D9 7.11 0.96 D17 0.00 6.15

TABLE 18 Example 6 • Aspherical Surface Data Surface Number S3 S4 KA 1.00000000E+00 1.00000000E+00 RA3 1.15512555E−03 −1.94623465E−03 RA4 4.63209518E−04 2.52796589E−03 RA5 1.00473917E−04 −3.74279507E−04 RA6 −2.62873609E−05 −1.43978882E−05 RA7 1.35555017E−06 4.10668149E−06 RA8 5.74392491E−09 5.42252724E−07 RA9 −2.60745641E−08 4.15486735E−09 RA10 5.11033586E−09 −1.45090144E−08 RA11 8.64355180E−10 −1.53424052E−09 RA12 −1.44079980E−10 2.60125627E−10 Surface Number S10 S11 KA 1.00000000E+00 1.00000000E+00 RA3 1.92172358E−03 1.98334763E−03 RA4 −1.18082835E−03 −3.45009857E−04 RA5 5.95776768E−04 4.03232975E−04 RA6 −8.55219828E−05 −2.20337853E−05 RA7 −4.72078410E−06 −9.21699036E−06 RA8 7.37744871E−07 1.20539081E−06 RA9 1.82849964E−07 −6.65042934E−08 RA10 5.54926203E−09 5.97488617E−09 RA11 −3.93487769E−10 3.82001970E−09 RA12 −2.92839034E−10 9.08656714E−11 RA13 3.11584436E−11 −1.56124185E−10 RA14 −1.47954864E−11 2.62156555E−11 RA15 −2.87211663E−12 1.04054388E−12 RA16 3.00829759E−13 −1.03639592E−12

Further, values corresponding to the conditional expressions (1-1), (1-2), (1-3), and (2) to (9) of Examples 1 to 6 are shown in Table 19. The values shown here are those of the conditions defined by each conditional expression, i.e., each literal expression. For example, values of |fw/f₁| are shown in the row of “Conditional Expression (2)”. As the conditional expressions (1-1), (1-2) and (1-3) commonly define the condition of fw/f_(G12), the values of fw/f_(G12) are indicated in the row collectively referred to as “Conditional Expression (1)”. Note that the values in Table 19 are those with respect to the d-line.

TABLE 19 Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 Conditional 0.019 0.015 0.056 −0.007 0.043 0.041 Expression (1) Conditional 0.409 0.453 0.409 0.409 0.410 0.412 Expression (2) Conditional 0.317 0.344 0.319 0.317 0.320 0.320 Expression (3) Conditional 0.774 0.758 0.778 0.775 0.781 0.777 Expression (4) Conditional −0.046 −0.033 −0.138 0.017 −0.104 −0.099 Expression (5) Conditional 0.373 0.345 0.324 0.445 0.284 0.234 Expression (8) Conditional 10.726 0.279 3.190 −20.460 3.298 2.204 Expression (7) Conditional 3.194 4.653 2.913 3.799 2.967 2.920 Expression (8) Conditional 2.374 2.006 2.386 2.369 2.372 2.369 Expression (9)

Spherical aberration, astigmatism, distortion (distortion aberration), and lateral chromatic aberration (chromatic aberration of magnification) of Example 1 at the wide angle end are illustrated respectively in diagrams A to D of FIG. 7 while spherical aberration, astigmatism, distortion (distortion aberration), and lateral chromatic aberration (chromatic aberration of magnification) of Example 1 at the telephoto end are illustrated respectively in diagrams E to H of FIG. 7.

Each aberration diagram is with respect to the d-line (wavelength or 587.6 nm), but the spherical aberration diagrams also illustrate aberrations with respect to the g-line (wavelength of 435.8 nm) and C-line (wavelength of 656.3 nm), and the lateral chromatic aberration diagrams illustrate aberrations with respect to the g-line and the C-line. In the astigmatism diagrams, the solid line illustrates astigmatism in the sagittal direction while the dotted line illustrates astigmatism in the tangential direction. The Fno. in the spherical aberration diagrams represents the F-number and ω in the other aberration diagrams represents the half angle of view.

Likewise, aberrations of Example 2 at the wide angle end and at the telephoto end are illustrated respectively in the diagrams A to H of FIG. 8. Likewise, aberration diagrams of Examples 3 to 6 are shown in FIGS. 9 to 12.

Next, an imaging apparatus according to an embodiment of the present invention will be described. As an example of imaging apparatus according to an embodiment of the present invention, a schematic configuration diagram of an imaging apparatus 10 that uses a zoom lens 1 of an embodiment, of the present invention is shown in FIG. 13. As for the imaging apparatus, for example, a surveillance camera, a video camera, or an electronic still camera may be cited.

The imaging apparatus 10 illustrated in FIG. 13 includes the zoom lens 1, an image sensor 2 disposed on the image side of the zoom lens 1 and captures an image of a subject formed by the zoom lens 1, a signal processing unit 4 that performs an arithmetic operation on the output signal from the image sensor 2, a zoom control unit 5 for zooming the zoom lens 1, and a focus control unit 6 for performing focus control. Note that a filter or the like may be disposed between the zoom lens 1 and the image sensor 2.

The zoom lens 1 has a first lens group G1 having a negative refractive power and is moved to the image side so as to draw a convex trajectory upon zooming from the wide angle end to the telephoto end, a second lens group G2 having a positive refractive power and is moved monotonically to the object side upon zooming from the wide angle end to the telephoto end, and a fixed aperture stop St. Note that each lens group is schematically illustrated in FIG. 13.

The image sensor 2 outputs an electrical signal by capturing an optical image formed by the zoom lens 1 and is disposed such that the imaging surface thereof corresponds to the image plane. As for the image sensor 2, for example, an image sensor formed of CCD, CMOS, or the like may be used.

Although not shown in FIG. 13, the imaging apparatus 10 may further include a shake correction mechanism that corrects blurring of captured image at the time of vibration or camera shake by moving, for example, a lens having a positive refractive power and constituting a part of the second lens group G2 in a direction perpendicular to the optical axis Z.

As the imaging apparatus 10 includes the zoom lens of the present invention having the advantageous effects described above, the apparatus may realize downsizing, low cost, and increased angle of view, as well as favorable optical performance.

So far, the present invention has been described by way of embodiments and Examples, but the present invention is not limited to the foregoing embodiments and Examples and various modifications may be made. For example, values of the radius of curvature of each lens element, surface distance, refractive index, Abbe number, aspherical surface coefficient, and the like are not limited to those illustrated in each numerical example and may take other values. 

What is claimed is:
 1. A zoom lens substantially consisting of a first lens group having a negative refractive power and a second lens group having a positive refractive power, disposed in order from the object side, wherein: zooming is performed by moving the first lens group and the second lens group; the first lens group is substantially composed of a first lens having a negative refractive power, a second lens, a third lens having a negative refractive power, and a fourth lens having a positive refractive power; the second lens group is substantially composed of four lenses; and when the focal length of the second lens from the object side in the first lens group is taken as f_(G12), the focal length of the entire system at the wide angle end is taken as fw the focal length of the first lens from the object side in the second lens group is taken as f_(G21), and the focal length of the second lens from the object side in the second lens group is taken as f_(G22), the zoom lens satisfies conditional expressions given below: the zoom lens satisfies a conditional expression given below: −0.04<fw/f _(G12)<0.17  (1-3), and 1.3<f _(G21) /f _(G22)<3.0  (9).
 2. The zoom lens of claim 1, wherein the zoom lens satisfies a conditional expression given below with respect to the focal lengths f_(G12) and fw: −0.04<fw/f _(G12)<0.12  (1-1).
 3. The zoom lens of claim 1, wherein the zoom lens satisfies a conditional expression given below with respect to the focal lengths f_(G12) and fw: −0.01<fw/f _(G12)<0.06  (1-2).
 4. The zoom lens of claim 1, wherein the zoom lens satisfies a conditional expression given below with respect to the focal lengths f_(G21) and f_(G22): 2.0<f _(G21) /f _(G22)<2.5  (9′).
 5. The zoom lens of claim 1, wherein the zoom lens satisfies a conditional expression given below when the focal length of the entire system at the wide angle end is taken as fw and the focal length of the first lens group is taken as f₁: 0.00<|fw/f ₁|<0.64  (2).
 6. The zoom lens of claim 5, wherein the zoom lens satisfies a conditional expression given below: 0.20<|fw/f ₁|<0.50  (2′).
 7. The zoom lens of claim 1, wherein the zoom lens satisfies a conditional expression given below when the focal length of the entire system at the wide angle end is taken as fw and the focal length of the second lens group is taken as f₂: 0.31<fw/f ₂<0.49  (3).
 8. The zoom lens of claim 7, wherein the zoom lens satisfies a conditional expression given below: 0.31<fw/f ₂0.35  (3′).
 9. The zoom lens of claim 1, wherein, when the focal length of the first lens group is taken as f₁ the focal length of the second lens group is taken as f₂, the zoom lens satisfies a conditional expression given below: 0.56<|f ₁ /f ₂|<1.04  (4).
 10. The zoom lens of claim 9, wherein the zoom lens satisfies a conditional expression given below: 0.70<|f ₁ /f ₂|<0.80  (4′).
 11. The zoom lens of claim 1, wherein, when the focal length of the first lens group is taken as f₁ and the focal length of the second lens from the object side in the first lens group is taken as f_(G12), the zoom lens satisfies a conditional expression given below: −0.19<|f ₁ /f _(G12)|<0.50  (5).
 12. The zoom lens of claim 11, wherein the zoom lens satisfies a conditional expression given below: −0.15<|f ₁ /f _(G12)|<0.30  (5′).
 13. The zoom lens of claim 1, wherein, when the maximum effective radius of the object side surface of the second lens from the object side in the first lens group is taken as H_(G12F), the radius of curvature of a spherical surface passing through the center of the object side surface of the second lens and a point on the object side surface at a height of H_(G12F) from the optical axis with the center of the object side surface as the apex of the spherical surface is taken as r′_(G12F), and the radius of curvature of a spherical surface passing through the center of the object side surface of the second lens and a point on the object side surface at a height of H_(G12F)×0.5 from the optical axis with the center of the object side surface as the apex of the spherical surface is taken as r″_(G12F), the zoom lens satisfies a conditional expression given below: 0.20<H _(G12F)×{(1/r′ _(G12F))−(1/r″ _(G12F))}  (6).
 14. The zoom lens of claim 13, wherein the zoom lets satisfies a conditional expression given below: 0.20<H _(G12F)×{(1/r′ _(G12F))−(1/r″ _(G12F))}<0.50  (6′).
 15. The zoom lens of claim 1, wherein, when the paraxial radius of curvature of the object side surface of the second lens from the object side in the first lens group is taken as r_(G12F) and the paraxial radius of curvature of the image side surface of the second lens from the object side in the first lens group is taken as r_(G12R), the zoom lens satisfies a conditional expression given below: 2.0<(r _(G12F) +r _(G12R))/(r _(G12F) −r _(G12R))<30.0  (7).
 16. The zoom lens of claim 15, wherein the zoom lens satisfies a conditional expression given below: 2.0<(r _(G12F) +r _(G12R))/(r _(G12F) −r _(G12R))<15.0  (7′).
 17. The zoom lens of claim 1, wherein, when the paraxial radius of curvature of the object side surface of the first lens from the object side in the first lens group is taken as r_(G11P) and the paraxial radius of curvature of the image side surface of the first lens from the object side in the first lens group is taken as r_(G11R), the zoom lens satisfies a conditional expression given below: 2.5<(r _(G11P) +r _(G11R))/(r _(G11P) −r _(G11R))<10.0  (8).
 18. The zoom lens of claim 17, wherein the zoom lens satisfies a conditional expression given below: 2.8<(r _(G11P) +r _(G11R))/(r _(G11P) −r _(G11R))<4.0  (8′).
 19. An imaging apparatus, comprising the zoom lens of claim
 1. 